For many years the most frequently used apparatus for cleaning metal objects and the like has consisted of degreasers operating with a heated stabilized chlorinated hydrocarbon, such as trichloroethylene or perchloroethylene, as the degreasing solvent. Such solvents boil at relatively high temperatures (188.6.degree. F. for trichloroethylene and 236.degree. F. for perchloroethylene) and, since one of the metal cleaning operations involves passage of the work through solvent vapors, there have been problems in some cases preventing the escape of toxic vapors to the atmosphere.
In the present invention, a low boiling solvent such as methylene chloride, trichlorotrifluoroethane, or azeotropes thereof, is employed as the degreasing solvent. These solvents boil at temperatures substantially lower than the boiling points of trichloroethylene or perchloroethylene. For example, methylene chloride boils at 103.6.degree. F., trichlorotrifluoroethane boils at 117.6.degree. F., and its azeotrope with methylene chloride boils at 97.2.degree. F. As used in the present application the term "low boiling solvent" refers to a metal degreasing solvent which boils in the range from about 97.degree. to about 120.degree. F. at normal atmospheric pressure.
In the operation of metal degreasing apparatus, maintenance of a solvent-vapor system is required so that a portion of the liquid solvent is continuously boiled in a boil chamber, thereby producing a vapor level overlying the liquid solvent. The vapor level is normally controlled by drawing off any vapors above the predetermined level and causing them to be condensed to liquid solvent, which may be subsequently recycled. By locating cooling coils in a cooling chamber adjacent to and communicating with the vapor containing space, the condensing action of the cooling coils is sufficient to draw off vapors above the vapor level.
Metal degreasing apparatus of the prior art has provided such a solvent-vapor system by use of heating coils immersed in the boil chamber and cooling coils positioned in the cooling chamber, both of which operate independently of one another and both of which require separate external energy inputs. This arrangement has proven to be both inefficient and wasteful in its use of energy. Furthermore, with the advent of the use of ultrasonic vibrators immersed in a portion of the liquid solvent, it has become necessary to control the temperature of that portion of the solvent to an optimum level for efficient operation of the ultrasonic transducers. This optimum temperature lies within the narrow range of from about 90.degree. to about 100.degree. F. This ultrasonic bath temperature control has required yet another external energy input and this further impairs efficiency.
While this inefficient and wasteful use of energy has been recognized by those skilled in the art, little, if any, improvement in efficiency has been made until that provided by the present invention.